High fatigue strength, combined with high tensile strength of 850 MPa or higher, is required in steel materials that are used in parts of marine diesel engines and diesel engines for power generation, in order to achieve higher engine outputs and make the engines more compact.
Herein, NiCrMo high-strength steel has been developed as steel for large steel forgings having such high tensile strength (see for instance Japanese Patent No. 3896365 and Japanese Patent No. 4332070). These steel grades exhibit high strength and high toughness.
The steel that is used in large crankshafts that are utilized for drive power transmission in vessels or the like is subjected, after forging and a thermal treatment, to machining for the purpose of finishing to a final shape. In this case, both high machinability and high polishability (ease of finishing) during machining are simultaneously required.
Forging steels for large crankshafts, however, have high strength, i.e. a tensile strength of 850 MPa or higher, and exhibit substantial cutting resistance. Accordingly, finishing to the final shape through machining is time-consuming, which detracts from productivity. It has been ordinarily very difficult to combine tensile strength of 850 MPa or higher with both excellent machinability and polishability, since cutting resistance increases proportionally to the strength (hardness) of the material.
It is an object of the present invention, arrived at in the light of the above issues, to provide a high-strength steel for steel forgings, and a steel forging, having high strength and boasting excellent machinability and polishability.